Thermoregulated poly(ethylene glycol) (PEG) biphase system, which is composed of PEG 4000 and the mixture of toluene and heptane, was developed and used in the hydroformylation of p-isobutylstyrene (IBS) catalyzed by Rh/OPGPP complex. Under the conditions of P = 5.0 MPa (H-2:CO = 1:1), P/Rh = 8 (molar ratio), reaction time = 5 h and temperature = 120 degrees C the conversion of IBS was 99% and the yield of aldehyde was 96%. The catalyst could be easily separated from the product by phase separation and reused for seven times without evident loss of activity. (c) 2006 Elsevier B.V. All rights reserved.